bpo-40268: Add _PyInterpreterState_GetConfig() (GH-19492)

Don't access PyInterpreterState.config member directly anymore, but
use new functions:

* _PyInterpreterState_GetConfig()
* _PyInterpreterState_SetConfig()
* _Py_GetConfig()
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 2709727..7052228 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -192,6 +192,13 @@
     PyInterpreterState *interp,
     _PyFrameEvalFunction eval_frame);
 
+PyAPI_FUNC(const PyConfig*) _PyInterpreterState_GetConfig(PyInterpreterState *interp);
+
+// Get the configuration of the currrent interpreter.
+// The caller must hold the GIL.
+PyAPI_FUNC(const PyConfig*) _Py_GetConfig(void);
+
+
 /* cross-interpreter data */
 
 struct _xid;